Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic-ultramafic intrusions, Eastern Desert, Egypt
Magmatic unmixing in spinel from late Precambrian concentrically-zoned mafic-ultramafic intrusions, Eastern Desert, Egypt
复制标题
DOI：
10.1016/j.lithos.2007.11.009
复制
发表时间：
2008-08
期刊：
影响因子：
3.5
通讯作者：
A. H. Ahmed;H. Helmy;S. Arai;M. Yoshikawa
中科院分区：
文献类型：
--
作者：
A. H. Ahmed;H. Helmy;S. Arai;M. Yoshikawa
尖晶石广泛存在于埃及东部沙漠晚前寒武纪镁铁质-超镁铁质杂岩的超镁铁质核部岩石中。这些复合物; Gabbro Akarem、Genina Gharbia和Abu Hamamid是阿拉斯加型杂岩的前寒武纪类似物，它们虽然蚀变较弱，但没有变质。每个侵入体都是由一个橄榄岩核心包裹的辉石和辉长岩在边缘。硅酸盐矿物学和化学研究表明，硅酸盐是由含水岩浆通过结晶分馏形成的。在辉石中记录了相对较高的Cr2 O3含量（高达1.1重量%）和角闪石（至多1.4重量%）三个plutons。Chrome尖晶石在熔体演化的不同阶段结晶，如早期橄榄石中的积云包裹体、辉石和角闪石中的包裹体以及晚期岩浆的积云间相。堆晶间Chrome尖晶石化学成分变化范围窄，主要为富Fe 3+尖晶石。单斜辉石和角闪石中的尖晶石包裹体显示出宽范围的Al（27-44 wt.% Al 2 O3）和Mg（6-13重量% MgO）含量，通常呈带状分布。这些尖晶石的不同化学性质反映了熔体演化的不同阶段和与寄主矿物的再平衡。早期堆晶Chrome尖晶石具有复杂的不混合结构和成分。三种类型的未混合尖晶石被确认;晶体取向，不规则和完全分离。解混产物是富Al（I型）和富Fe 3+（II型）尖晶石，在两个端元之间具有广泛的固溶体。未混合尖晶石的组成限定了相对于Cr-Al-Fe 3+的可熔性间隙，从Fe 3 +-Al接合处向Cr角延伸。尖晶石不混合响应于冷却和氧化态的增加而发生。初始尖晶石的化学性质和晶粒尺寸以及冷却速率控制了最终产物的分解类型和化学性质。晚岩浆阶段尖晶石不混合的原因与变质杂岩中尖晶石不混合的原因相似。未混合尖晶石的化学成分与初始尖晶石成分完全不同，在岩石成因解释中没有用处。来自氧化岩浆的尖晶石可能会在冷却过程中重新平衡，并且不是遗传考虑的好工具。
Spinel is widespread in the ultramafic core rocks of zoned late Precambrian mafic–ultramafic complexes from the Eastern Desert of Egypt. These complexes; Gabbro Akarem, Genina Gharbia and Abu Hamamid are Precambrian analogues of Alaskan-type complexes, they are not metamorphosed although weakly altered. Each intrusion is composed of a predotite core enveloped by pyroxenites and gabbros at the margin. Silicate mineralogy and chemistry suggest formation by crystal fractionation from a hydrous magma. Relatively high Cr2O3contents are recorded in pyroxenes (up to 1.1 wt.%) and amphiboles (up to 1.4 wt.%) from the three plutons. The chrome spinel crystallized at different stages of melt evolution; as early cumulus inclusions in olivine, inclusions in pyroxenes and amphiboles and late-magmatic intercumulus phase. The intercumulus chrome spinel is homogenous with narrow-range of chemical composition, mainly Fe3+-rich spinel. Spinel inclusions in clinopyroxene and amphibole reveal a wide range of Al (27–44 wt.% Al2O3) and Mg (6–13 wt.% MgO) contents and are commonly zoned. The different chemistries of those spinels reflect various stages of melt evolution and re-equilibration with the host minerals. The early cumulus chrome spinel reveals a complex unmixing structures and compositions. Three types of unmixed spinels are recognized; crystallographically oriented, irregular and complete separation. Unmixing products are Al-rich (Type I) and Fe3+-rich (Type II) spinels with an extensive solid solution between the two end members. The compositions of the unmixed spinels define a miscibility gap with respect to Cr–Al–Fe3+, extending from the Fe3+–Al join towards the Cr corner. Spinel unmixing occurs in response to cooling and the increase in oxidation state. The chemistry and grain size of the initial spinel and the cooling rate control the type of unmixing and the chemistry of the final products. Causes of spinel unmixing during late-magmatic stage are analogous to those in metamorphosed complexes. The chemistry of the unmixed spinels is completely different from the initial spinel composition and is not useful in petrogenetic interpretations. Spinels from oxidized magmas are likely to re-equilibrate during cooling and are not good tools for genetic considerations.